1. Field of the Invention
The invention relates to improvements in the manufacture of polythioether elastomers useful as sealants and more particularly it relates to an improved method of curing sulfhydryl terminated thioether polymers to obtain such elastomers. It is especially concerned with an improvement in curing such thioether polymers with an organic peroxy compound to provide thioether elastomers of enhanced thermal stability whereby the curing time is shortened and the effective working life of the curing reaction mixture is extended.
2. Description of the Prior Art
Sulfhydryl-terminated thioether polymers having an average functionality of 2 to 3 with respect to the sulfhydryl substituents, i.e. polymers with repeating alkylene monosulfide groups which contain 2 to 3 mercapto groups per polymer molecule, are the precursors of valuable elastomers useful as sealants, particularly architectural sealants are disclosed for example in I. G. Gardiner et al., U.S. Pat. No. 3,522,221, A. A. Oswald, U.S. Pat. No. 3,592,798, A. A. Oswald U.S. Pat. No. 3,717,618, my copending U.S. application Serial No. 501,716, filed Aug. 29, 1974, now abandoned, the continuation-in-part thereof, application Ser. No. 582,959, filed June 2, 1975 now Pat. No. 4,020,033, U.S. application Ser. No. 535,631, filed Dec. 23, 1974, to B. F. Dannels et al, now U.S. Pat. No. 3,992,289, issued Nov. 16, 1976, and the aforementioned U.S. Pat. No. 3,992,274.
Generally the sulfhydryl-terminated thioether polymers are prepared by polymerization of a dithiol or trithiol monomer and an unsaturated hydro-carbon containing at least one acetylenic bond or at least two ethylenic bonds per molecule.
As disclosed in the latter three applications the sulfhydryl terminated thioether polymers are generally liquid i.e. are at least fluid pastes, and can be cured by reaction with an inorganic or organic peroxy compound. The reaction is believed to involve oxidation of the terminal sulfhydryl, i.e. mercapto, substituents of the polymer to form disulfide linkages thereby cross linking the mono sulfide polymer chains to produce solid thioether elastomers. It is found that the inorganic peroxy compounds, such as lead dioxide, manganese dioxide and calcium peroxide, generally used in the preparation of these elastomers have several serious disadvantages which seriously limit their use as effective curing agents. For example, the inorganic peroxides are generally solids, insoluble in the liquid polythioether and hence they cannot be homogeneously dispersed in the liquid thioether polymer without prior comminution to a finely divided form and use of special dispersion equipment, e.g. paint mills, to evenly disperse the insoluble curing agent in the liquid thioether polymer substrate. Without the use of the foregoing inconvenient, expensive techniques and equipment, the inorganic peroxy compounds are unevenly dispersed in the liquid thioether polymers and elastomers obtained therefrom are unevenly cured resulting in serious structural weaknesses which limit their use as sealants, gaskets and the like.
Additionally during the curing reaction the inorganic peroxy compounds form inorganic salt residues. The latter remain in the cured elastomer and can catalyze degradation reactions in the cured product.
Moreover, many such inorganic peroxide compounds are highly colored, e.g. brown in the case of lead dioxide and black in the case of manganese dioxide, and introduce an undesirable coloration when employed as curing agents for the thioether elastomers. Accordingly, the use of lead dioxide and other colored inorganic peroxide compounds as curing agents is particularly undesirable where it is desired to prepare a substantially colorless thioether elastomer product or where it is desired to prepare a product of a particular color by use of a dye or pigmentary additive, i.e. said desired color in the product can be deleteriously affected in shade by the additional color introduced from the inorganic peroxy curing agent.
It is also known to use organic peroxy compounds for curing the liquid sulfhydryl-terminated thioether polymers to the corresponding thioether elastomers. Most of the known organic peroxy compounds have at least some degree of solubility in liquid sulfhydryl-terminated thioether polymers and hence their use as curing agents for the thioether polymers serves to overcome partially the disadvantages associated with the use of inorganic peroxy curing agents. However it has been discovered that the reaction of many organic peroxides with the liquid sulfhydryl-terminated thioether polymers is unacceptably slow. Additionally many organic peroxy compounds, on reaction with the thioether polymers, so rapidly convert the liquid polymer to a non-fluid liquid or paste which can not be worked or processed, that it is substantially impossible to process, i.e. mold, extrude or spread, the curing polymer mass after addition of the curing agent. In other words the organic peroxy reagents which are known to react with sulfhydryl compounds i.e. mercaptans, to form disulfide linkages on reaction with the present thioether polymers require extremely long time periods for curing the polymers or produce curing reaction masses which have insufficient effective working life, i.e. insufficient "pot-life". Moreover, many known organic peroxides when employed as curing agents for the liquid thioether polymers produce thermally unstable polymers. For example, U.S. Pat. No. 3,505,258 to J. R. Panek prescribes cumene hydroperoxide promoted by a copper salt in curing liquid sulfhydryl-terminated polysulfide polymers for extending the effective working life of the curing polymer mass. The liquid polysulfide polymers charged to the curing process of Panek are disulfide polymers and hence are structurally distinguished from the aforementioned liquid thioether polymers which are monosulfide polymers. The curing properties of such polysulfide polymers are also distinguished from those of the sulfhydryl-terminated thioether polymers since copper promoted cumene hydroperoxide provides curing masses of extremely insufficient effective working life when charged as curing agent to liquid thioether polymers devoid of disulfide linkages (as is illustrated by the results of Example 9 reported in Table 2 below). In addition to providing curing mixtures of inadequate pot-life copper-promoted cumene hydroperoxide also produces elastomers of insufficient thermal stability when employed as a curing agent for the liquid sulfhydryl-terminated monosulfide polymers as is illustrated by the results of Example 14 reported in Table 3 below.